It is known (see, for example, U.S. Pat. Nos. 3,931,518 (Miller) and 4,135,780 (Dyott), the disclosures of which are incorporated herein by reference, and the references cited therein, that it is possible to cause coupling of the guided modes (i.e. those modes that propagate in the core of an optical fiber) and the radiation modes (i.e. those modes that propagate in the cladding). These two patents referred to above make use of this mode coupling to enable the withdrawal of an optical signal from an optical fiber comprising a core and a cladding by first coupling the signal (or a part thereof) from the core into the cladding and then withdrawing the signal from the cladding by some suitable tap. This tap may be, for example, a pair of disks, one hard and one soft, between which the clad fiber is compressed (as in Miller), or a relatively large cross-section glass rod against which a straight section of the fiber lies and to which it is optically coupled by a liquid or jelly-like coupling material (as in Dyott).
The mode coupling is induced by the introduction of periodic microbends into the fiber, as is discussed in, for example, the Miller patent referred to above.
In U.S. Pat. No. 4,253,727 (Jeunhomme et al.), the disclosure of which is incorporated herein by reference, there is disclosed a technique for the extraction of light from, or injection of light into, a clad optical fiber in which both the mode coupling and signal extraction/injection is performed by a single device. This device comprises a longitudinally split glass cylinder having mode coupling means on the mating surfaces of the two halves between which the fiber is held and optical end surface(s) which are cones of revolution.
However, these techniques do not appear to have found wide application, and a possible explanation for this is that a fiber with the buffer removed, i.e. a fiber comprising only core and cladding, is much weaker than the same fiber with a buffer. This weakness manifests itself both as a much greater tendency to break when bent (a buffered fiber can stand much tighter bends and repeated bends better than an unbuffered one) and a tendency to stresscracking (because the hydrolytic attack on the unbuffered fiber is particularly prone to occur at bends where microcracks may be formed). Both of these factors limit the usefulness of any technique which operates on unbuffered fibers, especially when it is desired to be able to repeatedly connect and disconnect the optical coupler.
In U.S. patent application Ser. No. 437,054 filed Oct. 27, 1982 (Beals et al.), the disclosure of which is incorporated herein by reference, there is disclosed an optical fiber tap which does not interrupt the physical continuity of the tapped fiber. The tap, which is especially useful as a node in a ring topology data processing network comprises means for withdrawing signals from and introducing them into a buffered optical fiber through a microbend (this technique being disclosed in U.S. patent application Ser. Nos. 370,321, filed Apr. 21, 1982, abandoned; and continuation Application Ser. No. 437,053, filed Oct. 27, 1982 (Campbell et al.); the disclosures of which are incorporated herein by reference. The tap also comprises a signal attenuation means, located between the receiver and transmitter sections, which may be, for example, a means for introducing a series of microbends into the fiber.